Details from "Sound Opinions":
Klaus Voormann was an artist living in Hamburg when he followed the sound of live music down into a cellar one night and happened upon his first live rock and roll show. He saw two acts from Liverpool that night: Rory Storm and the Hurricanes (with Ringo Starr on drums) and an irreverent dance band called The Beatles.
The friendship he struck up with The Beatles would alter the course of his life and prove to be lifelong. When they leapt forward into psychedelia with Revolver, they turned to Voormann to create a fitting cover image. That work won a Grammy and a place of honor (in tattoo form) on Jim's arm.
When John Lennon started pulling away from The Beatles, he enlisted Voormann to play bass in the Plastic Ono Band. George Harrison and Starr followed suit, trusting their "A Hard Day's Night" era roommate, Voormann, to provide the bassline on many of their solo albums as well.
His stature as a session bass player grew throughout the 1970's- he can be heard on albums by Carly Simon, Harry Nilsson and Lou Reed among many others. In the early 1980s Voormann added“producer”to his resume through his work with the German band, Trio.